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Prevent monitor deadlocks with repeated restarts

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If a monitor is started and stopped before the external monitoring thread
has had time to start, a deadlock will occur.

The first thing that the monitoring threads do is read the monitor handle
from the monitor object. This handle is given as the return value of
startMonitor and it is stored in the monitor object. As this can still be
NULL when the monitor thread starts, the threads use locks to prevent
this.

The correct way to prevent this is to pass the handle as the thread
parameter so that no locks are required.
This commit is contained in:
Markus Mäkelä
2017-04-23 02:49:35 +03:00
parent a4e361b5e5
commit b434c94563
8 changed files with 38 additions and 37 deletions
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15
server/modules/monitor/mysqlmon/mysql_mon.c
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@ -246,7 +246,7 @@ startMonitor(MXS_MONITOR *monitor, const MXS_CONFIG_PARAMETER* params)
handle->id = config_get_global_options()->id;
handle->warn_failover = true;
handle->load_journal = true;
spinlock_init(&handle->lock);
handle->monitor = monitor;
}
/** This should always be reset to NULL */
@ -290,9 +290,13 @@ startMonitor(MXS_MONITOR *monitor, const MXS_CONFIG_PARAMETER* params)
MXS_FREE(handle);
handle = NULL;
}
else if (thread_start(&handle->thread, monitorMain, monitor) == NULL)
else if (thread_start(&handle->thread, monitorMain, handle) == NULL)
{
MXS_ERROR("Failed to start monitor thread for monitor '%s'.", monitor->name);
hashtable_free(handle->server_info);
MXS_FREE(handle->script);
MXS_FREE(handle);
handle = NULL;
}
return handle;
@ -1090,8 +1094,8 @@ void do_failover(MYSQL_MONITOR *handle, MXS_MONITOR_SERVERS *db)
static void
monitorMain(void *arg)
{
MXS_MONITOR* mon = (MXS_MONITOR*) arg;
MYSQL_MONITOR *handle;
MYSQL_MONITOR *handle = (MYSQL_MONITOR *) arg;
MXS_MONITOR* mon = handle->monitor;
MXS_MONITOR_SERVERS *ptr;
int replication_heartbeat;
bool detect_stale_master;
@ -1101,9 +1105,6 @@ monitorMain(void *arg)
int log_no_master = 1;
bool heartbeat_checked = false;
spinlock_acquire(&mon->lock);
handle = (MYSQL_MONITOR *) mon->handle;
spinlock_release(&mon->lock);
replication_heartbeat = handle->replicationHeartbeat;
detect_stale_master = handle->detectStaleMaster;